In advanced technology nodes of integrated circuit industry, high k dielectric material and metal are adopted to form a gate stack of a field-effect transistor (FET) such as a metal-oxide-semiconductor field-effect transistors (MOSFETs). In the existing method to pattern the metal gate stack, due the film uniformity, the work function of a metal layer in the metal gate stack is shifted toward the mid-gap at the gate edge. Accordingly, the threshold voltage is unexpectedly changed, leading to poor short channel control at the constant sub-threshold leakage current. Additionally, the drive current is degraded due to the increase of source drain extension resistance. Furthermore, the poor uniformity of the metal films in the existing method leads to the worsen work function variation within the gate. Therefore, a structure of a metal gate stack and a method making the same are needed to address the issues identified above.